Lasers are widely used for the purposes of precision fabrication, such as the micromachining of metals, ceramics, crystals and glasses. In many laser micromachining applications, pulsed lasers are used to remove a controlled amount of material with each pulse. In particular, by varying the location and energy of each delivered pulse, a net shape can be generated.
The accuracy of net shape generation by pulsed laser precision micromachining, for example the fabrication of micro-optics in fused silica, depends on the accuracy and stability of the energy of the delivered pulses. In such applications, material removal depth control typically needs to be ˜0.5% or better, imposing a similar requirement on the accuracy and stability of pulse energy. Accuracy and stability of beam pointing are also critical, as beam pointing typically determines the location at which the laser spot is incident on the workpiece, which in turn affects the spatial distribution of material removed and hence the net shape generated. Spectral stability is also important since the source wavelength can directly affect the amount of material removed, and can also affect the properties of optical components in the system. In particular, when an acousto-optic modulator (AOM) is used to modulate pulse energy in a first-order diffracted beam, a change in source spectrum will lead to a change in diffraction angle and hence modify the spot landing position. This can lead to an elongated or misplaced laser spot on the workpiece, thereby affecting the net shape generated. This effect can, in principle, be avoided by taking the 0th order beam as the output, but this has this disadvantage that due to the limited contrast of the AOM the output beam can never be switched fully off.
An additional consequence of using an AOM is that there is an additional, unwanted, component of beam deflection that varies with RF drive power. This results in an undesirable cross-coupling between AOM transmission, which is used to control pulse energy, and beam pointing, which affects where the laser spot lands on the workpiece.
Many continuous wave (CW) lasers exhibit temporal power fluctuations greater than 2%. Pulsed lasers typically exhibit pulse-to-pulse energy fluctuations of a similar magnitude.
Attempts to address these problems have been made. For example, closed loop control of the laser drive can reduce the pulse to pulse variability in pulse energy, but in many cases this technique does not provide sufficient pulse energy stability to achieve precision micromachining, and is difficult to implement in practice, since it often requires a more complex laser power supply. Also, this approach does not deal with fluctuations in laser spectrum and laser beam pointing.
US patent application publication US2005/0270631 A1 proposes the use of closed-loop control to generate output pulses of fixed energy from a laser source with fluctuating pulse energy. This approach calculates a modulator control signal for attenuation of a given pulse using measurements of previous output pulse energies. This closed-loop control approach requires a high degree of correlation of pulse energy from pulse to pulse, as it cannot correct for pulse energy fluctuations that are not temporally correlated.
U.S. Pat. No. 4,277,146 proposes a means for limiting the peak instantaneous power of an individual laser pulse, using a Pockels cell. This method only starts to attenuate the beam when it exceeds a preset threshold level, and does not affect the transmitted power as long as it remains below this threshold level. While this approach does control the maximum peak power of the beam for each individual pulse, it does not determine the output pulse energy. This approach requires closed-loop control of laser power within the individual pulse, and so requires a modulator whose response time is very much shorter than the laser pulsewidth. The stability of closed loop control systems is very sensitive to delay in the loop filter, and so while this approach is well-suited to use with fast modulators, such as the Pockels cell, it is less well-suited to use with modulators such as AOMs, where the time delay due to acoustic propagation is often significant on the timescale of the laser pulse.
US patent application publication US2005/0270631 A1 also proposes a means for stabilising laser beam pointing using two or more AOMs, or a compound AOM incorporating two orthogonal diffracting acoustic fields. This approach has the disadvantage that the use of multiple Bragg diffractions is expensive, complex, and reduces system efficiency. This approach also requires complex electronics to modulate AOM drive frequency as part of the control system, which increases system cost.
In one specific example, pulsed carbon dioxide (CO2) lasers are used to fabricate fused silica micro-optics by removing material from an initially flat fused silica blank, to generate a net surface shape with a specific optical function, for example a lens. In another example, continuous wave (CW) CO2 lasers have also been used to fabricate micro-optics.
A wavelength-stabilised CW CO2 laser may be amplitude modulated to generate pulses. However, while such a laser exhibits a stable spectrum, fairly low noise and fairly stable pointing, the high mean power through subsequent optical components can cause thermal lensing and the pulses may have inconveniently low peak power.
Alternatively, RF-excited pulsed CO2 lasers are lower cost, more robust and widely available, and may be configured to exhibit high peak power and low mean power. However, for applications such as laser precision micro-machining, such lasers exhibit an unstable spectrum, poor pulse-to-pulse energy repeatability and significant pulse-to-pulse beam pointing variation.
In both cases, the best external modulator is typically an AOM. However, it is know that varying the RF drive power to an AOM typically steers the beam, which can cause spot landing errors at the workpiece. Hence these approaches have a requirement for pulse energy stabilization, beam pointing control, spectral stabilisation.
It is therefore an object of at least one embodiment of the present invention to provide a method of generating a laser pulse that obviates and mitigates one or more of the disadvantages and limitations of the prior art.
Moreover, it is therefore an object of at least one embodiment of the present invention to provide an apparatus to generate a laser pulse that obviates and mitigates one or more of the disadvantages and limitations of the prior art.